Systems treating a material flow of solid matter, which comprise one or more unloading points where one or more unloaders unload solid matter from storage to one or more conveyors which convey said matter to a feeding point where said solid matter is either actively guided with separate feeding devices away from the conveyor and forwards in the process or where the solid matter passively discharges from the conveyor, are used in connection with a large number of different processes. Such systems treating a material flow of solid matter are used in, for instance, treating processes of the raw material in chemical pulp manufacture, in fuel feeding in power plants using solid fuel and in rock-crushing plants. Chemical pulp manufacture utilizes one or more chip kinds or grades, which are usually stored outdoors in heap or clamp storage. From such storage, the chips are unloaded to belt conveyors, which convey the chips to a chip silo functioning as intermediate storage, from which chips are fed at a desired speed to a chemical pulp cooking process. In power plants using solid matter, a fuel, such as peat or chips, is unloaded from storage to conveyors, which convey the fuel to the fuel feeding of power boilers. The fuel feeding to power boilers can be implemented in a plurality of ways, for example in such a way that there is a separate conveyor for the feed of each power boiler, or that fuel is fed to several power boilers with one conveyor, in which case separate feeding devices are arranged in connection with the conveyor for feeding fuel to each power boiler. In rock-crushing plants, the rock material is unloaded either directly in the quarrying location or from a heap serving as intermediate storage to the conveyor, which transfers the quarry-run rock to a crusher, from where the crushed material is transferred forwards.
A prerequisite for the efficient operation of all the above-mentioned and similar processes is that the amount of solid matter to be fed from the conveyor system onwards to solid matter intermediate storage or the amount of solid matter to be fed to a process treating solid matter is right. For example, from the viewpoint of appropriate operation of a chemical pulp cooking process, it is important for the surface of the chip silo to remain at correct level or at least within an allowed variation range. In order to guarantee efficient energy production, the amount of fuel to be fed to the boiler, in turn, must be neither too small nor too large. Likewise, the amount of quarry-run rock to be fed to a rock-crushing process must be sufficiently large in order for the crusher to operate at full power, but still sufficiently small so as not to clog the crusher.
The amount of chips to be fed to a chip silo of a chemical pulp cooking process has conventionally been controlled by adjusting the speed of chip unloading from clamp or heap storage to a belt conveyor with a level controller based on measuring the surface level of the chip silo. However, the problem with this solution is that there is a delay corresponding to the whole length of the conveyor system in the control circuit of the level controller, whereby the control circuit of the chip feeding is naturally very slow. There are also solutions in which the conveyor system is equipped with control drives, whereby not only the speed of the chip unloading but also the speed of the conveyors can be controlled with the same message arriving from the level controller and being based on measuring the chip silo surface level. However, due to control-driven conveyors, the delay length of the conveyor system varies, whereby the chip flow from the conveyor system after the delay does not correspond to the chip flow at the beginning of the conveyor system. In addition, when using an unloader moving in parallel in the direction of travel of the conveyor belt, the variation of the chip unloading point causes a changing delay. The problem with these solutions in use is further that disturbances in chip unloading, such as variation in th e quality of chips coming form the storage area, due to clods caused by the freezing of chips, for example, are carried along through the whole conveyor system without being weakened. Further, with some apparatus combinations and performance requirements, a satisfactory or even stable control solution cannot be found to control the amount of chips to be fed to the chip silo.
In chip feeding, it is also known to use 3-point control, whereby both a level controller and a flow rate controller are used for controlling the amount of chips to be fed to the chip silo. The level controller determines the difference variable between the surface level of the chips in the silo and the set value of the surface level, and forms an output variable on the basis this, the output variable being fed as one input variable to the flow rate controller. The output variable of the flow rate controller is used to adjust the speed of the chip unloading and/or the speed of the conveyors. A second input variable of the flow rate controller is formed by the difference between the chip flow incoming to the silo and the chip flow discharging from the silo. The chip flow discharging from the silo can be determined based on the speed of the unloader at the bottom of the silo. However, the chip flow incoming to the silo at each particular moment of time is not known very accurately due to the variety of problems mentioned above, and therefore it is not possible to implement accurate control of chip feeding with 3-point control either.
Further, SU publication 984 487 discloses a method for stabilizing the amount of material in the crusher chamber of a crusher on the basis of the amount of crushed material and the amount of material going to the crusher. SU publication 697 981 discloses a method for controlling fuel feeding in coal-burning power plants where fuel is fed to a feed conveyor from several different silos. According to the method, the aim is to regulate the amount of fuel fed from different silos in such a way that the fuel surfaces in different silos would stay at the same level at the same time as the total amount of fuel fed to the feed conveyor is kept desired. Further, a method is known in which the aim is to stabilize the amount by weight of the fuel fed via a conveyor. According to the solution, the amount by weight of the fuel on the conveyor is calculated on the basis of the weight of the fuel on the conveyor belt and the speed of the conveyor. If the fuel weight varies, for instance when the fuel moisture or density changes, the speed of the conveyor is changed so that the amount by weight of the fuel to be fed remains constant. A prerequisite for the functioning of the method is that the conveyor is provided with load cells, by means of which the weight of the fuel to be fed is continuously measured.